The decoy is a ship fitting of two antennas on folded panels, built in an Assembler, that echoes a ship's emissions back louder than they are. To sensors that make out only mass, the ship carrying it reads as several times heavier, and its signature, how easily sensors pick it up, rises by the same factor. Sensors that can tell the ship's class see its true mass.
It is a small fitting for a slot, not a hardpoint. It is fitted and removed at a station with a shipyard in the ship's system.